Yoselyn Ortega, 55, is charged with murdering 6-year-old Lucia Krim, who went by Lulu, and 2-year-old Leo Krim in their Upper West Side home in 2012.

"This trial will be very hard for us and hard for a lot of you," said the victims' father Kevin Krim on Facebook, where the parents of Lulu and Leo Krim were doing their very best to push forward these days in the positive, with a fund for creative art they've started in their children's names.

But the reality of the sight of the nanny came crashing down on their mother, Marina Krim, who broke down before she even made it to the witness stand.

In tears and glaring at her former nanny, she told the court, "I'm sorry, I just need a look..You are out of this world."

In the moments before that, opening statements from both the prosecution and the defense left no doubt about what Ortega did that day.

"In the 90 minutes during which the Krims entrusted her to care for and protect their children, the defendant brutally butchered both children, slitting their throats," said Assistant District Attorney Courtney Groves.

The defense, meanwhile, is aiming to prove that Ortega suffered from untreated mental illness for most of her life.

"The evidence will show that she has a corroborated history of hearing voices and disassociating from reality since the age of 16," said defense attorney Valerie Van Leer-Greenberg.

But on Thursday, nothing sunk in more than the broken-hearted mom, teetering as she described how she and her daughter Nessie frantically raced back to their apartment building on the Upper West Side, checking every empty room until they found Nessie's younger brother and older sister, and their bloody nanny, in the bathroom.

"Nessie screams a high pitched 3-year-old scream...and I look at the defendant, and I say, 'I hate you'," said Marina Krim.

Ortega faces a possible sentence of life behind bars. She is currently being held without bail.